PCI-compatible programmable logic devices

ABSTRACT

A programmable logic integrated circuit device has several features which help it perform according to the PCI Special Interest Group&#39;s Peripheral Component Interface (“PCI”) signaling protocol. Regions of programmable logic within the device are closely coupled to the data signal output pins and clock signal input pins such that delay between application of a clock signal to the device and output of a data signal from the device is within PCI signal standards for delay. The device also includes output circuitry that can be configured to selectively invert signals to output enable and data input enable terminals of the output circuitry.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of United States provisional patentapplication No. 60/038,289, filed Feb. 20, 1997. This application isalso a division of application Ser. No. 08/919,988, filed Aug. 28, 1997,which is hereby incorporated by reference herein in its entirety.

BACKGROUND OF THE INVENTION

This invention relates to programmable logic integrated circuit devices,and more particularly to constructing and operating such devices so thatthey are compatible with the PCI Special Interest Group's PeripheralComponent Interface (“PCI”) bus signaling protocol.

The PCI Special Interest Group's PCI bus signaling protocol has becomewidely accepted. At present the PCI standard is a 32 bit bus with a 33MHZ clock and stringent requirements regarding TCO (time from clock tooutput: no more than 11 nanoseconds), TCZ (time from clock to highimpedance: no more than 11 nanoseconds), TSU (time for setup: no morethan 7 nanoseconds), and THD (hold time: no more than 0 nanoseconds). Tomeet the PCI standard a device must therefore be able to (1) output datavery rapidly following a PCI clock signal (TCO), (2) release the PCI busvery rapidly following a PCI clock signal (TCZ), (3) set up to inputdata very shortly before a PCI clock signal (TSU), and (4) require datato remain present no longer than arrival of a PCI clock signal (THD).

Programmable logic devices have not generally been designed to meet thePCI standard, and it is accordingly difficult or impossible for mostsuch devices to meet that standard. It is therefore difficult orimpossible for most programmable logic devices to interface with a PCIbus. This is a limitation on the usefulness of programmable logicdevices which is becoming increasingly important as the PCI standardbecomes more widely used.

In view of the foregoing, it is an object of this invention to provideprogrammable logic devices which meet PCI bus standards.

SUMMARY OF THE INVENTION

This and other objects of the invention are accomplished in accordancewith the principles of the invention by providing programmable logicdevices having at least some registers that are relatively closelycoupled to data signal input/output pins of the device. For example,there is relatively little signal switching between (1) the input andoutput terminals of these registers and (2) the data input/output pinsof the device. The clock signal input terminals of these registers arealso relatively closely coupled to the clock signal input pin of thedevice (i.e., again there is little or no signal switching between theclock signal input pin of the device and the clock signal inputterminals of these registers). These registers preferably supply bothoutput data and output enable signals to tri-state drivers that drivethe input/output pins. These characteristics help the device meet thePCI TCO and TCZ requirements. Programmable delay may be provided betweeninput/output pins of the device and the data signal input terminals ofadjacent registers to compensate for clock signal skew (e.g., from oneside of the device to the other). This helps the device meet the PCI TSUand THD requirements.

Further features of the invention, its nature and various advantageswill be more apparent from the accompanying drawings and the followingdetail description of the preferred embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic block diagram of a representativeportion of an illustrative programmable logic integrated circuit deviceconstructed in accordance with this invention.

FIG. 2 is a simplified schematic block diagram of an illustrativeembodiment of portions of the FIG. 1 device.

FIG. 3 is a simplified schematic block diagram of an illustrativeembodiment of other representative portions of the FIG. 1 device.

FIG. 4 is a simplified schematic block diagram of an illustrativeembodiment of still other representative portions of the FIG. 1 device.

FIG. 5 is a simplified block diagram of an illustrative PCI networkwhich can include programmable logic devices constructed in accordancewith the invention.

FIG. 6 is a simplified block diagram of an illustrative system employinga programmable logic device in accordance with the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The representative portion of illustrative programmable logic device 10shown in FIG. 1 is the lower right-hand corner of that device. Device 10includes plural regions 20 of programmable logic disposed on the devicein several intersecting rows and columns of such regions. Thus the threerepresentative regions 20 shown in FIG. 1 are the three right-mostregions in the bottom row, and the bottom-most region in each of thethree right-most columns of regions 20.

Each region 20 includes a plurality of subregions of programmable logic30. In the particular example shown in FIG. 1 each subregion 20 includesten subregions 30. (The terms region and subregion are used purely asrelative terms, and other relative terms could be used if desired. Forexample, regions 20 could be referred to as super-regions 20 andsubregions 30 could be referred to as regions 30.) Each subregion 30 isprogrammable to perform any of several relatively elementary logicfunctions on several data input signals 40 applied to that subregion.For example and as shown in FIG. 2, each subregion 30 may include afour-input look-up table 212 which is programmable to produce an outputsignal that is any logical combination of the four inputs 40 to thelook-up table. Each subregion 30 may additionally include a register 220(e.g., a flip-flop) and programmable logic connectors (“PLCs”) 222 a and222 b (e.g., switches) for allowing the look-up table output signal tobe either directly output by the subregion or registered by the registerand then output by the subregion. Flip-flop 220 has a data inputterminal D, a clock input terminal CLK, and a data output terminal Q.Each of the local (“L”) and global (“G”) outputs of each subregion 30may be either the combinatorial (unregistered) or registered output ofthe subregion. Drivers 224 a and 224 b amplify these signals. Theillustrative subregion structure shown in FIG. 2 is somewhat simplified,and a more fully featured embodiment of a suitable subregion structureis shown in Cliff et al. U.S. patent application Ser. No. 08/902,416,filed Jul. 29, 1997, which is hereby incorporated by reference herein inits entirety.

Each row of regions 20 has an associated plurality of inter-regioninterconnection conductors 50. In the illustrative embodiment shown inFIG. 1 each group of conductors 50 includes 96 global horizontalconductors that extend along the entire length of the associated row, 48half horizontal conductors that extend along each left or right half ofthe length of the associated row, and four so-called fast conductorsthat extend along the entire length of the associated row and that arealso part of a network that extends along all rows of device 10.

Each column of regions 20 has an associated plurality of inter-regionconductors 60. In the illustrative embodiment shown in FIG. 1 each groupof conductors 60 includes 20 global vertical conductors that extendalong the entire length of the associated column.

Each horizontally adjacent pair of regions 20 is interspersed with localinterconnection conductors 70 of various types. Certain localinterconnection conductors 70 are also provided at each end of each row.For example, each group of local interconnection conductors includes 22region feeding conductors 70 a. Each of conductors 70 a is programmablyconnectable to any one of several of the conductors 50 that it crosses.These programmable logic connectors (“PLCs”) are indicated by thecircles 72 a in FIG. 1. Thus conductors 70 a are usable to bring signalsfrom adjacent conductors 50 into the regions 20 to the left and right ofthose conductors 70 a. Inverting drivers 74 a are provided alongconductors 70 a for strengthening the signals in those conductors.Inverters 76 a are level restorers for drivers 74 a.

The local outputs of the subregions 30 that are adjacent to each groupof conductors 70 are connected to conductors 70 b in that group.Conductors 70 b are usable to apply the local outputs to selected onesof conductors 50 and 60 via PLCs 80 and 82, respectively. The upperportions of some of conductors 70 b are alternatively usable to makeconnections from conductors 50 to conductors 60 (and/or to otherconductors 50) via conductors 84, Inverters 86, PLCs 88, and PLCs 82(and/or PLCs 80). Inverting drivers 71 b are provided in conductors 70 bto strengthen the signals on those conductors.

The global outputs of the subregions 30 that are adjacent to each groupof conductors 70 are connected to conductors 70 c in that group.Conductors 70 c are usable to apply the global outputs to selected onesof conductors 50 and 60 via PLCs 90 and 92, respectively. The upperportions of conductors 70 c are alternatively usable to make connectionsfrom conductors 60 to conductors 50 (and/or other conductors 60). Theseconnections are made via PLCs 94, inverters 96, PLCs 98, and PLCs 90(and/or PLCs 92). Inverting drivers 70 c are provided in conductors 70 cto strengthen the signals on those conductors.

Each subregion 30 has two main data input conductors 40 coming from theleft and two main data input conductors 40 coming from the right. Eachof conductors 40 is programmably connectable via PLCs to any of theconductors 70 a and 70 b that it crosses. PLCs are not provided betweenconductors 70 c and 40. Conductors 40 can therefore bring data signalsinto the associated subregions 30 from the conductors 70 a and 70 b oneach side of the associated subregion.

At each end of each row of regions 20 is a plurality of input/outputpins 100. In the embodiment shown in FIG. 1, for example, there are teninput/output pins 100 at each end of each row of regions 20. For use asan output pin each of pins 100 has an associated tri-state driver 102.Each tri-state driver 102 has a data input terminal and an output enableterminal. When the signal applied to the output enable terminal of atri-state driver is low, the driver is tri-stated (i.e., has a highimpedance at its output terminal). When the signal applied to the outputenable terminal of a tri-state driver is high, the driver is enabled todrive the signal applied to its data input terminal to its outputterminal. The data input terminal of each tri-state driver 102 is fedfrom the output terminal of an associated PLC 104. Each PLC 104 isprogrammable to select either the true or the complement of the signalon an associated lead 108. (The complement signal is produced by anassociated inverter 106.) Each of leads 108 is programmably connectableby PLCs to any of the conductors 70 a, 70 b, and 70 c that it crosses.The output enable input terminal of each tri-state driver 102 is fedfrom the output terminal of another associated PLC 110. Each PLC 110 isprogrammable to select either the true or the complement of the signalon an associated lead 114. (The complement signal is produced by anassociated inverter 112.) Each of leads 114 is programmably connectableby PLCs to any of the conductors 70 a, 70 b, and 70 c that it crosses.

For use as an input pin each of pins 100 is connectable to several ofthe adjacent conductors 50 via drivers 120 and 122 and PLCs 124. (Thefast conductors in group 50 may be excluded from these connections.)

At each end of each adjacent pair of columns of regions 20 there aremore input/output pins 130. In the embodiment shown in FIG. 1, forexample, there are two input/output pins 130 at each end of each column.For use as an output pin each of pins 130 has an associated tri-statedriver 132. Each tri-state driver 132 has a data input terminal and anoutput enable terminal and operates in the same way that has beendescribed above for tri-state drivers 102. The data input terminal ofeach tri-state driver 132 is fed from the output terminal of anassociated PLC 134. Each PLC 134 is programmable to select either thetrue or the complement of the signal on an associated lead 138. (Thecomplement signal is produced by an associated inverter 136.) Each ofleads 138 is programmably connectable by PLCs to any of the conductors70 a and 70 b that it crosses. The output enable input terminal of eachtri-state driver 132 is fed from the output terminal of anotherassociated PLC 140. Each PLC 140 is programmable to select either thetrue or the complement of the signal on an associated lead 144. (Thecomplement signal is produced by an associated inverter 142.) Each ofleads 144 is programmably connectable by PLCs to any of the conductors70 a and 70 b that it crosses.

For use as an input pin each of pins 130 is connectable to several ofthe adjacent conductors 60 via drivers 150 and 152 and PLCs 154.

Conductors 170 (two of which are provided along each side of device 10)extend to all of the rows of regions 20 on the device. The same is truefor conductors 180, two of which are provided along each side of device10. Each of conductors 170 receives an input signal from an associatedfast input pin (not shown in FIG. 1 but shown representatively at 210 inFIG. 3) of device 10. Each of conductors 180 receives a signal from anassociated conductor like conductors 108 in a row near the verticalcenter of the device. PLCs 172 and drivers 174 and 176 allow the signalson conductors 170 and 180 to be selected and applied to the fastconductors in each group of conductors 50.

FIG. 3 shows that device 10 may have 22 columns and six rows of regions20. FIG. 3 further shows that a fast input pin 210 on device 10 may beused to receive a clock input applied to the device. For example, thisclock signal may be the PCI clock signal. FIG. 3 still further showsthat the fast conductor network (including conductors 170 and fastconductors 50) may be used to distribute this clock signal to the clockinput terminals of registers 220 in at least some of the subregions 30in various regions 20. Ignoring, for the moment, elements 230 and 232,FIG. 3 also shows that conductors 50, 70, and 40 allow data signalsapplied to pins 100 (used as input pins) to be applied to the data inputterminals of registers 220. (FIG. 3 does not show the look-up tablelogic 212 shown in FIG. 2, but it will be understood that data reachesthe data input terminal of each register 220 in FIG. 3 via such logic212.) The data input signals shown in FIG. 3 may be PCI bus datasignals.

There is inevitably some delay in transmitting a data signal from a datapin 100 to the D input terminal of a flip-flop 220. FIG. 3 shows that tominimize and standardize such delay, it is good practice to have eachdata pin 100 feed the D input terminal of a flip-flop 220 that isrelatively close to that pin 100. The clock signal, on the other hand,may be applied to a fast input 210 that is relatively close to some ofthese flip-flops, but relatively far from others of these flip-flops. Inthe example shown in FIG. 3, the fast input pin 210 that is used for theclock signal is relatively close to the flip-flop 220 on the right, butrelatively far from the flip-flop on the left.

The PCI bus specification provides that a data signal may be availableas little as 7 nanosecond prior to a clock signal transition, and thatthe data signal may end as little as 0 nanoseconds after that clocksignal transition. The first of these parameters is TSU. The secondparameter is THD. If for some actual PCI data the hold time isrelatively short (i.e., at or near the 0 nanosecond minimum), the timerequired for the clock signal to travel through device 10 to flip-flops220 that are relatively distant from the clock input pin 210 may begreat enough that THD will have expired at such a flip-flop before theclock signal transition can reach that flip-flop. A flip-flop 220 thatthus receives its clock signal somewhat delayed due to propagation delayon device 10 may therefore fail to register data signals with relativelyshort hold time.

To reduce the risk of this happening, data input paths that may be usedfor PCI data include programmable delay circuits such as are shown inFIG. 3. In the illustrative embodiment shown in FIG. 3, eachprogrammable delay circuit comprises elements 230 and 232. Elements 230are multiple inverters (e.g., 230 a and 230 b) connected in series todelay the data signal passing through them from the associated datainput pin 100 to one input terminal of the associated PLC 232. A secondparallel connection between the data input pin 100 and the other inputterminal of the PLC 232 does not include any delay elements andtherefore does not significantly delay the data signal passing alongthat path. Each PLC 232 is programmable to select either the delayed orundelayed version of the data signal from the associated data pin 100.The selected version of the data signal is passed on to the remainder ofdevice 10.

The delay chains of elements 230 are typically used for data pins 100and associated registers 220 that are relatively remote from the clockinput pin. The delay chains of elements 230 are typically not used fordata pins 100 and associated registers 220 that are relatively close tothe clock input pin. In the particular example shown in FIG. 3, thedelay chain of elements 230 would be used for the data input pin 100 andregister 220 on the left, but the undelayed path (parallel to the delaychain of elements 230) would be used for the data input pin 100 andregister 220 on the right.

The amount of delay available using a delay chain of elements 230 maydiffer depending on other parameters of device 10. However, a typicaldelay that can be provided by a chain of elements 230 may be about 3nanoseconds. Given the relatively small minimum value of THD specifiedby the PCI bus standard (i.e., minimum THD=0), it may be desirable todesign device 10 so that all data is delayed somewhat more than clocksignals. On the other hand, it is not desirable for this delay to be toolarge because it slows down device response in all applications.Moreover, such data delay should not be so great that it exceeds therelatively small minimum value of TSU (i.e., 7 nanoseconds), becausethen the flip-flop may be clocked before the data arrives at theflip-flop, thereby preventing the flip-flop from registering the data.

In sum, for data pins 100 and associated registers 220 that are disposedon device 10 relatively close to clock input pin 210, the delay chainsof elements 230 are not used because the clock signal will arrive at theregisters between the minimum times for TSU and THD. Indeed, in theseinstances, using the delay chains of elements 230 could cause theregisters to be clocked ahead of the arrival of the data at theregisters when the data has TSU at or near the minimum value (i.e., 7nanoseconds). For data pins 100 and associated registers that aredisposed on device 10 relatively far from clock input pin 210, the delaychains of elements 230 are used to bring the data back into the propertime relationship to the clock signal as the data and clock signals areapplied to the registers. In particular, this additional data delayensures that the data arrives at these registers at a time that isbetween the minimum values for TSU (i.e., 7 nanoseconds) and THD (i.e.,0 nanoseconds).

The foregoing is just one example of how programmable delay chains canbe used in accordance with this invention to help ensure that dataarrives at registers of a programmable logic device within the PCI TSUto THD time interval relative to arrival of a clock signal transition atthose registers. Instead of basing the decision as to whether or not touse each programmable delay chain on only distance of the register fromthe clock input pin, that decision can alternatively or additionally bebased on such other factors as distance of the register from the datainput pin, loading (and therefore speed) of the conductors between theclock and/or data input pins and the register, loading (and thereforespeed) of the switching (such as PLCs 72 a) between the clock and/ordata input pins and the register, and any other relevant operatingcharacteristics of the device as those operating characteristics applyto the timing of the arrival of clock and data signals to particularregisters. For convenience herein and in the appended claims, all suchcharacteristics may be referred to as clock or data signal propagationtiming characteristics. Thus each programmable delay chain of elements230 can be either used or not used to ensure arrival of the associateddata signal and a clock signal at a desired register within the PCI TSUto THD time interval depending on the data and clock signal propagationtiming characteristics applicable to the transmission of the data andclock signals to that register.

If a data input pin does not drive an input register, then the delaychain of elements 230 for that input pin can be turned off to achievefaster speed because there are no TSU or THD concerns.

Compatibility with the above-considered PCI standards for input purposesis also facilitated by having data pins 100 and 130 more directlyconnected to global conductors 50 and 60, respectively. As shown in FIG.4, for example, there is preferably no switching (other than forprogrammable delay elements 230 and 232) of input signals from pins 100prior to drivers 122. The outputs of drivers 122 can then be switchedonto any of several conductors 50 by PLCs 124 (controlled byprogrammable function control elements (“FCEs”) 125). (FIG. 4 also showsa typical FCE 233 for controlling an associated PLC 232.)

With regard to meeting the PCI specification for output signal timing(i.e., TCO and TCZ), structures of the type shown in FIG. 1 areparticularly advantageous because the subregions 30 near the peripheryof device 10 are very closely coupled to the adjacent pins 100 and 130.For example, there is relatively little switching between thesesubregions 30 and the adjacent pins 100 and 130. The switching that isprovided, however, has the ability to locally gram active high or activelow for either data or output enable of tri-state drivers 102. It is notnecessary to use other subregions 30 to provide any of these options.One subregion 30 can be used to provide the output enable signal for allthe output pins 100/130 served by the region 20 that includes thatsubregion. These various structural features help device 10 meet the PCIstandards for TCO and TCZ. In particular, these structural features helpdevice 10 output data or a high impedance within 11 nanoseconds after aPCI bus clock signal. In other words, within TCO=11 nanoseconds after aPCI clock signal is applied to device 10, the register 220 in asubregion 30 adjacent a pin 100 or 130 can receive that clock signal andcan respond to that clock signal by beginning to output data L or Gwhich passes through the output stage switching and drivers of thedevice to those adjacent pins 100 or 130. Similarly, within TCO (orTCZ)=11 nanoseconds after a PCI clock signal is applied to device 10,the register in a subregion 30 adjacent a pin 100 or 130 can receivethat clock signal and can respond to that clock signal by beginning tooutput data L or G which passes through the output stage switching toenable or tri-state the drivers 102 serving those adjacent pins 100 or130.

The foregoing advantages of device 10 are preferably provided withoutthe need for specially constructed input/output subregions 30. In otherwords, the subregions 30 that are closely associated with input/outputpins 100/130 are preferably the same as or not significantly differentfrom other subregions 30 on the device. These subregions are thereforefully available to perform logic like all the other subregions. It isnot necessary to devote a portion of the area of device 10 to circuitrythat is specially adapted to meet the PCI bus standard.

The provision of elements such as 104, 106, 110, and 112 for horizontaloutput pins 100, and such as 134, 136, 140, and 142 for vertical outputpins 130 has another important benefit. These elements allow any unusedoutput pin to be programmed high (logic 1), low (logic 0), or tri-stated(high impedance). If an output pin is not used, then conductors 108 and114 (for a horizontal output pin 100) or 138 and 144 (for a verticaloutput pin 130) will be pulled high by default because no PLC isprogrammed to apply a signal to those conductors. (The default couldalternatively be low rather than high.) By appropriately programming theassociated elements 104/106/110/112 or 134/136/140/142, these defaulthigh (or low) signals can be used to cause the associated output pin 100or 130 to be high, low, or tri-stated. It is not necessary to use otherelements such as conductors 70 a to apply particular signals to unusedoutput pins 100 or 130. Such waste of other valuable resources istherefore avoided.

FIG. 5 shows a typical PCI bus network 300. This network includes a PCImaster device 310 and any number of PCI slave devices 320 a, 320 b, etc.Devices 310 and 320 are interconnected via a 32 bit PCI data bus 312, aPCI control bus 314, and a PCI clock bus 316. PCI master 310 typicallyoriginates the PCI clock and control signals on buses 314 and 316. Databus 312 is typically bi-directional. The features of device 10 (FIGS.1-4) that are described above facilitate use of device 10 for any partor all of any of elements 310 and 320. Because device 10 can be PCIcompatible, device 10 can interface directly with the various PCI buscomponents shown in FIG. 5 and can therefore serve as PCI master element310, as PCI slave elements 320, or as any portions of any of thoseelements.

FIG. 6 illustrates a programmable logic device 10 of this invention in adata processing system 402. Data processing system 402 may include oneor more of the following components: a processor 404; memory 406; I/Ocircuitry 408; and peripheral devices 410. These components are coupledtogether by a system bus 420 and are populated on a circuit board 430which is contained in an end-user system 440. Bus 420 may be or includea PCI bus, or bus 420 may employ PCI-type signaling.

System 402 can be used in a wide variety of applications, such ascomputer networking, data networking, instrumentation, video processing,digital signal processing, or any other application where the advantageof using reprogrammable logic is desirable. Programmable logic device 10can be used to perform a variety of different logic functions. Forexample, programmable logic device 10 can be configured as a processoror controller that works in cooperation with processor 404. Programmablelogic device 10 may also be used as an arbiter for arbitrating access toa shared resource in system 402. In yet another example, programmablelogic device 10 can be configured as an interface between processor 404and one of the other components in system 402. It should be noted thatsystem 402 is only exemplary, and that the true scope and spirit of theinvention should be indicated by the following claims.

The PLCs mentioned throughout this specification (which includes theappended claims) can be implemented in any of a wide variety of ways.For example, each PLC can be a relatively simple programmable connectorsuch as a switch or a plurality of switches for connecting any one ofseveral inputs to an output. Alternatively, each PLC can be a somewhatmore complex element which is capable of performing logic (e.g., bylogically combining several of its inputs) as well as making aconnection. In the latter case, for example, each PLC can be productterm logic, implementing functions such as AND, NAND, OR, or NOR.Examples of components suitable for implementing PLCs are EPROMS,EEPROMS, pass transistors, transmission gates, antifuses, laser fuses,metal optional links, etc. The components of PLCs can be controlled byvarious, programmable, function control elements (“FCEs”), which are notalways shown separately in the accompanying drawings. (With certain PLCimplementations (e.g., fuses and metal optional links) separate FCEdevices are not required.) FCEs can also be implemented in any ofseveral different ways. For example, FCEs can be SRAMs, DRAMs, first-infirst-out (“FIFO”) memories, EPROMS, EEPROMs, function control registers(e.g., as in Wahlstrom U.S. Pat. No. 3,473,160), or the like. From thevarious examples mentioned above it will be seen that this invention isapplicable both to one-time-only programmable and reprogrammabledevices.

It will be understood that the foregoing is only illustrative of theprinciples of the invention and that various modifications can be madeby those skilled in the art without departing from the scope and spiritof the invention. For example, the particular logic elements and groupsof logic elements that have been shown for performing various functionsare only illustrative, and other logically equivalent structures can beused instead if desired. The use of look-up tables for performing thebasic logic of the subregions is also only illustrative, and thesubregions can instead be implemented in other ways, in which othertypes of logic are performed. For example, the subregions could includesum-of-products logic implemented using EPROM devices. The use ofserially connected inverters 230 to produce delay is only illustrative,and any other suitable delay-producing elements can be used instead ifdesired. The programmable delay networks of elements 230 and 232 can beconstructed with more than two delay options if desired. For example,each such network could include several delay chains having differentamounts of delay in addition to the undelayed path, and PLC 232 could beprogrammable to select any of these chains or the undelayed path. Thiswould increase the number of available delay options.

What is claimed s:
 1. A programmable logic device comprising: aplurality of regions of programmable logic, each of which includes aregister having a clock signal input terminal, a data signal inputterminal, and a data signal output terminal and configured to respond toa clock signal applied to the clock signal input terminal by applying tothe data signal output terminal a data output signal indicative of thesignal concurrently applied to the data signal input terminal; a clocksignal input pin connected to the clock signal input terminals of theregisters; a plurality of tri-state drivers, each having a data signalinput connection, a data signal output connection, and an output enablesignal connection and being configured to respond to a signal applied tothe output enable signal connection by applying to the data signaloutput connection a signal indicative of the signal concurrently appliedto the data signal input connection or a high impedance depending on thesignal applied to the output enable signal connection; programmablelogic connector circuitry configured to selectively connect the dataoutput signal terminals of the registers to the data signal input andoutput enable signal inputs of the tri-state drivers; and a plurality ofdata signal output pins, each of which is connected to the data signaloutput connection of a respective one of the tri-state drivers, whereinthe signal path length from the clock signal input pin to each registerclock input signal terminal and from each register data signal outputterminal through the programmable logic connector circuitry and thetri-state drivers to the data signal output pins is such that delaybetween application of a clock signal to the clock signal input pin andapplication of a data output signal from a register to a data signaloutput pin is within the PCI TCO time interval.
 2. The device defined inclaim 1 wherein the programmable logic connector circuitry is configuredto allow the data signal output terminal of any of the registers to beconnected to the data signal input connection of any of the tri-statedrivers.
 3. The device defined in claim 1 wherein the programmable logicconnector circuitry is configured to allow the data signal outputterminal of any of the registers to be connected to the output enablesignal input connection of any of the tri-state drivers.
 4. The devicedefined in claim 1 wherein the registers are a subset of similarregisters on the device, the registers in the subset being the closestof all of the registers to the data signal output pins.
 5. The devicedefined in claim 1 wherein the programmable logic connector circuitryincludes: programmable inversion circuitry associated with each of thetri-state drivers and configured to selectively apply either the true orcomplement of a data output signal from a register to the data signalinput connection of the associated tri-state driver.
 6. The devicedefined in claim 1 wherein the programmable logic connector circuitryincludes: programmable inversion circuitry associated with each of thetri-state drivers and configured to selectively apply either the true orcomplement of a data output signal from a register to the output enablesignal input connection of the associated tri-state driver.
 7. Thedevice defined in claim 1 further comprising: a plurality ofinterconnection conductors for conveying data signals to groups of theregions of programmable logic on the device; and a driver circuitassociated with each of the data signal output pins configured to applyto a respective interconnection conductor a signal applied to theassociated output pin without passing that signal through any switchesbetween the output pin and the driver circuit.
 8. A digital processingsystem comprising: processing circuitry; memory circuitry coupled to theprocessing circuitry; and a device as defined in claim 1 coupled to theprocessing circuitry and the memory circuitry.
 9. A printed circuitboard on which is mounted a device as defined in claim
 1. 10. Theprinted circuit board defined in claim 9 further comprising: memorycircuitry mounted on the printed circuit board and coupled to saiddevice.
 11. The printed circuit board defined in claim 9 furthercomprising: processing circuitry mounted on the printed circuit boardand coupled to said apparatus.
 12. Output circuitry for a programmablelogic device, which includes programmable circuitry, comprising: atri-statable driver having a data input terminal, a data outputterminal, and an output enable input terminal, and configured to respondto an output enable signal applied to its output enable input terminalby applying to its data output terminal a signal indicative of a datainput signal applied to its data input terminal when the output enablesignal has a first logical state and by applying to its data outputterminal a high impedance when the output enable signal has a secondlogical state; first and second leads respectively configured to act assources for the data input and output enable signals; a firstprogrammable circuit operatively connected between the first lead andthe data input terminal and configured to selectively invert a signal onthe first lead for application to the data input terminal; and a secondprogrammable circuit operatively connected between the second lead andthe output enable input terminal and configured to selectively invert asignal on the second lead that has come from the programmable circuitryfor application to the output enable input terminal.
 13. The circuitrydefined in claim 12 further comprising: an output pin of theprogrammable logic device operatively connected to the data outputterminal.
 14. The circuitry defined in claim 12 wherein the firstprogrammable circuitry comprises: an inverter having an input connectedto the first lead and an output, and configured to apply to its output asignal which is the logical inverse of a signal applied to its input;and a programmable logic connector having a first input connected to thefirst lead, a second input connected to the output of the inverter, andan output operatively connected to the data input terminal of thetri-statable driver, and configured to select a signal applied to eitherof its first or second inputs for application to its output.
 15. Thecircuitry defined in claim 12 wherein the second programmable circuitrycomprises: an inverter having an input connected to the second lead andan output, and configured to apply to its output a signal which is thelogical inverse of a signal applied to its input; and a programmablelogic connector having a first input connected to the second lead, asecond input connected to the output of the inverter, and an outputoperatively connected to the output enable input terminal of thetri-statable driver, and configured to select a signal applied to eitherof its first or second inputs for application to its output.